Isoelectric Point Calculator

The isoelectric point, abbreviated pI (sometimes written pH(I) or IEP), is the pH at which a protein or peptide carries no net electrical charge. At this specific pH the number of protonated positive groups exactly balances the number of deprotonated negative groups, so the molecule is electrically neutral overall even though individual side chains remain charged. The isoelectric point calculator on this page computes that pH directly from an amino acid sequence, so you can predict protein behavior before you ever step into the lab.

Why does the pI matter so much? At its isoelectric point a protein reaches its minimum solubility, because there is no net charge to drive electrostatic repulsion between molecules, and proteins tend to aggregate and precipitate. In an electric field a molecule at its pI will not migrate, which is the entire physical basis of isoelectric focusing and two-dimensional gel electrophoresis. When the surrounding pH is below the pI the protein carries a net positive charge, and when the pH is above the pI it carries a net negative charge. This single number therefore tells you the sign of the charge at any working pH, which is essential for choosing buffers, ion-exchange resins, and purification strategies.

Proteins rich in acidic residues such as aspartate and glutamate have low pI values, while proteins rich in basic residues such as lysine and arginine have high pI values. Most cytoplasmic proteins cluster in the slightly acidic-to-neutral range, which is why so many sit just below physiological pH. The protein pI calculator makes these trends concrete by reporting the exact net charge at pH 7.0 and at physiological pH 7.4 alongside the pI itself.

This calculator works entirely from the primary amino acid sequence you paste into the input box. First it cleans the text, converting it to uppercase and stripping out any character that is not one of the twenty standard one-letter amino acid codes (A, R, N, D, C, E, Q, G, H, I, L, K, M, F, P, S, T, W, Y, V). It then counts the seven ionizable side chains that influence charge: aspartate (D), glutamate (E), cysteine (C), and tyrosine (Y) on the acidic side, and histidine (H), lysine (K), and arginine (R) on the basic side. The free N-terminus and C-terminus of the chain are always included as one additional positive and one additional negative group.

For every ionizable group the calculator applies the Henderson-Hasselbalch equation to find the fraction of molecules that carry charge at a given pH. Positive groups contribute a charge of +1 / (1 + 10^(pH - pKa)) each, while negative groups contribute -1 / (1 + 10^(pKa - pH)) each. Summing all of these contributions gives the net charge of the protein at that pH.

To find the pI, the tool runs a binary search across the pH range from 0 to 14. It evaluates the net charge at the midpoint, and if the charge is still positive it moves the lower bound up, otherwise it moves the upper bound down. It repeats this bisection until the interval narrows to within 0.001 pH units, which pinpoints the pH where net charge crosses zero. The same charge function is then sampled across the full pH range to draw the charge-versus-pH curve and to report the net charge at pH 7.0 and pH 7.4.

The accuracy of any isoelectric point calculator depends on the set of pKa values it uses for the ionizable groups. Different reference tables differ by a few tenths of a pH unit, which is why pI predictions from different tools can disagree slightly. This calculator uses a single fixed, well-established set of values, shown below, applied identically to every residue of that type.

Notice that cysteine and tyrosine are treated as weakly acidic groups: their thiol and phenol hydroxyl protons ionize at relatively high pH, so they only begin to carry negative charge in alkaline conditions. Histidine, with a pKa near 6.0, is the only side chain whose charge changes meaningfully right around physiological pH, which is why histidine residues are so often found in enzyme active sites that switch between protonated and deprotonated states.

The calculator reports three headline numbers: the isoelectric point (pI), the net charge at pH 7.0, and the net charge at the physiological pH of 7.4. Reading them together gives a complete charge profile. If your protein's pI is below 7, the molecule already carries a net negative charge at neutral pH, and the net-charge readouts will confirm this with negative values. If the pI is above 7, the protein is net positive at neutral pH.

The relationship is simple and reliable: at any pH below the pI the protein is positively charged, at any pH above the pI it is negatively charged, and exactly at the pI the net charge is zero. The charge-versus-pH curve plotted on the page visualizes this crossover, with blue points marking positive net charge and red points marking negative net charge. The point where the curve crosses the horizontal axis is the pI.

For practical work, the magnitude of the charge matters as much as the sign. A protein with a net charge of several units at your working pH will bind strongly to an oppositely charged ion-exchange resin and is unlikely to aggregate, whereas a protein whose net charge is close to zero is near its pI and may precipitate. Because the calculator counts only side chains and termini and applies fixed pKa values, the predicted pI is a sequence-based estimate; the experimentally measured pI can shift due to the three-dimensional fold, buried residues, post-translational modifications, and local electrostatic environments. Treat the result as a strong starting hypothesis for buffer and purification design rather than an exact measured constant.

Knowing a protein's isoelectric point is a daily necessity across biochemistry, molecular biology, and bioprocessing. In protein purification, the pI tells you whether to use a cation or anion exchange column at a given buffer pH: load below the pI and the protein binds a cation exchanger, load above the pI and it binds an anion exchanger. Choosing a buffer pH one to two units away from the pI keeps the protein soluble and well charged for binding and elution.

In isoelectric focusing (IEF) and two-dimensional gel electrophoresis, proteins migrate through a pH gradient until they reach the position where the local pH equals their pI and they stop moving. Predicting pI values in advance helps you select the correct pH range for immobilized pH gradient strips and interpret which spot corresponds to which protein. The pI also predicts where a protein sits in an SDS-PAGE or native gel relative to others.

Beyond electrophoresis, pI values guide crystallization screens (proteins often crystallize best near their pI where solubility is lowest), antibody and biologic formulation (where charge governs viscosity and stability), and the design of fusion tags and charge-engineering mutations. Researchers studying enzymes use pI and per-residue charge to reason about catalytic mechanisms, while structural biologists use net charge to predict electrostatic surface potentials. Whenever you need to predict how a protein behaves in a buffer, an electric field, or a chromatography column, the isoelectric point is one of the first numbers worth calculating.